Support query params and fragment in redirect URL

Content store has now been changed to [support anchor fragments and query parameters](https://github.com/alphagov/content-store/pull/93).

However the Unpublishing presenter currently [strips these off when sending them to content store](https://github.com/alphagov/whitehall/blob/master/app/presenters/publishing_api_presenters/unpublishing.rb#L59).

This change ensures that the anchor fragment and query parameters are preserved.

We use URI::Generic.build to compose the pieces of the URI, rather than trying to do anything too clever with regexps.

Trello story:
https://trello.com/c/sMBwhDUN/98-alternate-url-of-unpublishings-should-support-anchor-fragment-and-query-parameters